The present invention relates to a correlation calculation control circuit and a correlation calculation control method.
In the global positioning system (GPS) which is one type of satellite positioning system, GPS satellite signals are respectively transmitted from GPS satellites which orbit the earth, and a GPS receiver calculates the present position based on the received GPS satellite signals.
The GPS satellite transmits a radio wave in the L1 band (1575.42 MHz) and a radio wave in the L2 band (1227.6 MHz). Commercial GPS receivers are permitted to use only the GPS satellite signal in the L1 band. The GPS satellite signal in the L1 band is a spread spectrum signal which is modulated by a coarse/acquisition (C/A) code (pseudo random noise (PRN) signal which differs depending on the satellite) and a navigation message. The navigation message includes an almanac (approximate orbit information of all the GPS satellites), an ephemeris (precise orbit information of the GPS satellite), time information, and the like.
The GPS receiver acquires/tracks the received GPS satellite signal, and decodes the navigation message included in the GPS satellite signal. The GPS receiver then calculates the present position based on the orbit information of the GPS satellite and the time information included in the decoded navigation message. Specifically, the GPS receiver calculates the position of each GPS satellite when the GPS satellite has transmitted the GPS satellite signal and the pseudo-range from the GPS receiver to each GPS satellite from the difference between the time at which each GPS satellite has transmitted the GPS satellite signal and the time at which the GPS receiver has received the GPS satellite signal. The GPS receiver calculates the present position by solving simultaneous equations in which the three-dimensional position of the GPS receiver and the clock error between the GPS satellite and the GPS receiver are unknown quantities. The GPS receiver can calculate its three-dimensional position by receiving the GPS satellite signals from at least four GPS satellites. This is because each coordinate value of the three-dimensional position and the clock error between the GPS satellite and the GPS receiver are used as unknown quantities.
The GPS receiver acquires the GPS satellite signal by performing correlation calculations on the received signal and a C/A code replica. The C/A code replica is a signal which simulates the C/A code included in the acquisition target GPS satellite signal and is pseudo-generated by the GPS receiver. Specifically, the GPS receiver performs a coherent process which calculates the correlation between the C/A code included in the received signal and the C/A code replica (code replica) using FFT calculations, and an incoherent process which integrates the correlation values (results of the coherent process) to calculate the integrated correlation value. As a result, the phases of the C/A code and a carrier frequency contained in the GPS satellite signal are obtained (see JP-A-11-242075, for example).
In the coherent integration process, a phenomenon may occur in which the integrated correlation value is saturated (i.e., a memory which stores the integrated value overflows). When saturation of the integrated values has been detected, a related-art GPS receiver temporarily suspends coherent integration (i.e., suspends integration of the correlation values) until the present integration time expires. However, it may be desirable that integration of the correlation values not be suspended depending on the cause of saturation.
For example, the integrated values may be saturated when the strength of the received signal is high (strong signal) or when cross-correlation has occurred. When the integrated values are saturated due to a strong received signal, no problem occurs even if integration of the correlation values is suspended, since the peak value of the integrated value has reached a value sufficient for determination of the phase of the C/A code and the like.
The term “cross-correlation” refers to a phenomenon in which not only the correlation between the C/A code of the acquisition target GPS satellite signal included in the received signal and the C/A code replica, but also the correlation between a signal component other than the C/A code and the C/A code replica is detected. When saturation has occurred due to cross-correlation, the integrated value includes a plurality of peak values. In this case, whether the detected saturation is saturation due to the peak value of the C/A code or saturation due to the peak value of the signal component other than the C/A code cannot be determined at this point. Specifically, the peak value of the C/A code of the acquisition target GPS satellite signal may be insufficient when saturation has been detected. In this case, the GPS satellite signal is not accurately acquired if integration of the correlation values is suspended. This increases the period of time required for positioning calculations (particularly initial position calculations). Therefore, it is desirable that integration of the correlation values not be suspended when saturation due to cross-correlation has occurred.